BEIJING - China's national observatory on Sunday renewed a yellow alert for heavy fog in many parts of the country.

Thick fog is expected to hit parts of Liaoning, Jilin, Shandong, Henan, Anhui, Jiangsu, Jiangxi and Hunan through Sunday morning, according to the National Meteorological Center (NMC).

Visibility in some affected areas will be reduced to less than 200 meters, the NMC said.

The observatory also forecast heavy fog in the Liaodong Gulf and the Yellow Sea on Sunday, which will reduce visibility in some areas to less than 1 km.

The NMC said drivers should slow down to safe speeds, and airports, freeways and ports should take appropriate safety measures.

China has a four-tier color-coded warning system for severe weather, with red being the most serious, followed by orange, yellow and blue.